The present invention relates generally to electrochemical electrode constructions, and particularly to ultramicroelectrode structures and methods of constructing such ultramicroelectrode structures.
An "ultramicroelectrode" may be defined as an electrode which has at least one dimension which is 10 .mu.m or less. Thus, for example, an ultramicroelectrode may be a disk with a radius less than or equal to 10 .mu.m, or a ring having its electrically conductive width less than or equal to this dimension. Alternatively, the ultramicroelectrode could be a band of undetermined length, which has a width less than or equal to 10 .mu.m. In contrast, a "microelectrode" typically has its dimensions on the order of one to several millimeters.
Ultramicroelectrodes are useful for several reasons. Firstly, electrochemical experiments can be conducted in highly electrically resistive media. Secondly, chemical or electrochemical processes which are too fast for larger electrodes can be studied using an ultramicroelectrode. Additionally, the mass transport rate to an ultramicroelectrode is higher than that can be achieved using larger electrodes. Furthermore, lower detection limits can be achieved using an ultramicroelectrode, due to its higher signal-to-background ratio. Finally, steady-state signals, as opposed to transient signals can often be obtained with ultramicroelectrodes.
While ultramicroelectrodes provide enhanced analytical sensitivity in comparison with other electrode structures, one disadvantage of the ultramicroelectrode is that only very small currents can be obtained at such a minuscule electrode. However, this problem can be solved by constructing an ultramicroelectrode array or ensemble. In this regard, the term array is sometimes used to refer to assemblies where a plurality of ultramicroelectrodes are evenly spaced from each other, while the term ensemble is sometimes used to indicate that the ultramicroelectrode elements are not necessarily evenly spaced from each other. In any case, the total measured current from such ultramicroelectrode assemblies is the sum of the currents obtained at each of the ultramicroelectrode elements in the assembly.
One pertinent example of an ultramicroelectrode ensemble is discussed in "Preparation and Electrochemical Characterization of Ultramicroelectrode Ensembles", by Reginald M. Penner and Charles R. Martin, Anal. Chem. 1987, 59, 2625-2630. This article discloses a procedure for preparing ultramicrodisk electrode ensembles, and is hereby incorporated by reference. Specifically, this article discloses a method in which platinum is electrochemically deposited into the pores of a microporous polycarbonate host membrane until the platinum layer begins to overgrow the surface of the host membrane. The surface of this composite membrane is then impregnated with polyethylene by immersion, and the polyethylene and excess platinum are subsequently removed by polishing. This ultramicrodisk membrane is then stretched over a convex electrode, and held in place with a sleeve of heat shrinkable Teflon tubing. The host membrane used in this procedure is a Nuclepore .RTM. polycarbonate membrane from Nuclepore, Inc.
It is a principal objective of the present invention to provide an ultramicroelectrode assembly and method of construction which does not require an electrochemical deposition step, nor requires the use of precious metals in the construction.
It is also a principal objective of the present invention to demonstrate that an ultramicroelectrode assembly can yield lower electroanalytical detection limits.
It is another objective of the present invention to provide a quick and inexpensive method of making an ultramicroelectrode assembly.
It is an additional objective of the present invention to provide an ultramicroelectrode assembly which is capable of demonstrating all three of the theoretically predicted electrochemical response limiting cases.
It is a further objective of the present invention to provide an ultramicroelectrode assembly which exhibits low capactive currents.
To achieve the foregoing objectives, the present invention provides an ultramicroelectrode assembly which generally comprises an electrically nonconducting host membrane having a plurality of micro-sized pores extending through the membrane, and a macro-sized substrate electrode in contact with this host membrane. The host membrane has its pores impregnated with an electrically conductive medium, and the substrate electrode is in electrical contact with the impregnated pores of the host membrane. The electrically conductive medium is a carbon paste, and the substrate electrode includes a confined volume of such carbon paste in contact with an interior surface of the impregnated membrane.
The method of making an ultramicroelectrode assembly according to the present invention includes the steps of preparing the carbon paste, forcing the carbon paste into and through the pores of the host electrode, and joining the impregnated membrane to the substrate electrode. This assembly may also be heated in an oven to improve the adhesion between the impregnated membrane and the carbon paste substrate electrode.
Additional advantages and features of the present invention will become apparent from a reading of the detailed description of the preferred embodiments which makes reference to the following set of drawings in which: